Laser-based optical communication systems offer high bandwidth communication through either free-space or optical fiber mediums or channels. Many of these systems encode information using intensity modulation (e.g. on/off keying). One potential weakness of these systems is that an eavesdropper with access to the communication channel is able to receive the same information flow as the intended receiver. While the eavesdropper may not be able to decrypt the message, the fact that the eavesdropper has access to the same information as the intended receiver presents a security concern.
In some cases, there is an additional concern that the eavesdropper could intercept the message and send a different message to the intended receiver. This is known as “spoofing.” Depending on the quality of the spoof message, it may be difficult for the intended receiver to distinguish between it and a real message. Another tactic the eavesdropper could use to impede the flow of information is saturating the receiver's detector by illuminating the detector with high intensity, in-band laser light. This is known as “blinding.” The term “jamming” is a generic term that refers to techniques that impede information flow through spoofing, blinding, or some other means.
Other techniques or approaches that rely on optical channels to pass information between platforms have the same potential security and jamming concerns. Two such examples are LIDAR and laser target designation. In both of these instances, a system may be especially vulnerable to these weaknesses because these techniques inherently rely on the receiver detecting light that has reflected off an object. If the object has an in-band detector (which is especially possible in military applications) and the information is encoded using intensity modulation, then the detector at the object will receive the same encoded waveform as the intended receiver. This received information may be decrypted or used to spoof the intended receiver. For example, in the case of laser target designation, a modulated retroreflector may be used to change the laser waveform that the missile detector receives.
It is the object of the present disclosure to overcome the above-mentioned potential weaknesses associated with conventional laser-based optical communication systems by using a reference locked optical system that uses frequency tuning, hoping, and encoding techniques to both provide additional message security and protect the channel against jamming.